# 4.1 变量 (Variables)

Variables in Sway are *immutable by default*. This means that, by default, once a variable is declared, its value cannot change. This is one of the ways how Sway encourages safe programming, and many modern languages have this same default. Let's take a look at variables in detail.

Sway中的变量默认是 *不可变 immutable* 的。这意味着，默认情况下，一旦一个变量被声明，其值就不能改变。这是Sway鼓励安全编程的方式之一，许多现代语言也有同样的默认。让我们来看看变量的详细情况。

### 声明一个变量 (Declaring a Variable)

Let's look at a variable declaration:

让我们看一下变量声明：

```
let foo = 5;

```

Great! We have just declared a variable, `foo`. What do we know about `foo`?

很好! 我们刚刚声明了一个变量，`foo`。我们对`foo`有什么了解？

1. It is immutable.\
   它是不可变的。
2. Its value is `5`.\
   它的值是`5`。
3. Its type is `u64`, a 64-bit unsigned integer.\
   它的类型是`u64`，一个64位无符号整数。

`u64` is the default numeric type, and represents a 64-bit unsigned integer. See the section [Built-in Types](https://fuellabs.github.io/sway/v0.38.0/book/basics/built_in_types.html) for more details.

`u64`是默认的数字类型，代表一个64位无符号整数。更多细节见[内置类型](https://fuellabs.github.io/sway/v0.38.0/book/basics/built_in_types.html)一节。

We can also make a mutable variable. Let's take a look:

我们也可以制作一个可变的变量。让我们看一下：

```
let mut foo = 5;
foo = 6;

```

Now, `foo` is mutable, and the reassignment to the number `6` is valid. That is, we are allowed to *mutate* the variable `foo` to change its value.

现在，`foo`是可变的，重新分配给数字`6`是有效的。也就是说, 我们被允许 *mutate* 变量`foo`来改变它的值。

### 类型注释 (Type Annotations)

A variable declaration can contain a *type annotation*. A type annotation serves the purpose of declaring the type, in addition to the value, of a variable. Let's take a look:

一个变量声明可以包含一个 *类型注释*。类型注释的作用是，除了变量的值之外，还要声明其类型。让我们来看看：

```
let foo: u32 = 5;

```

We have just declared the *type* of the variable `foo` as a `u32`, which is an unsigned 32-bit integer. Let's take a look at a few other type annotations:

我们刚刚声明了变量`foo`的\_类型为`u32`，是一个无符号的32位整数。让我们来看看其他一些类型注释：

```
let bar: str[4] = "sway";
let baz: bool = true;

```

If the value declared cannot be assigned to the declared type, there will be an error generated by the compiler.

如果声明的值不能被分配给声明的类型，编译器会产生一个错误。